One way to handle the line ordering is to have each worker send its output to a different file. Have each line preceded with the line number where it resided in the original file. Then just merge the files: 
sort  --field-separator=,
      --key=1,1
      --numeric-sort
      --merge
      worker_output_*
| cut --delimiter=,
      --fields=2-

[download]

--merge is key.

In reply to Re: How do you parallelize STDIN for large file processing? by ikegami
in thread How do you parallelize STDIN for large file processing? by forsaken75

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